Lawyers for two British nationals who face the death penalty in Pakistan, after allegedly being tortured to give confessions for murder they insist they did not commit, have asked the Foreign Office to intervene urgently in the case.

Naheem Hussain and his friend Rehan Zaman, both 24, were arrested nearly five years ago over a land dispute in Kashmir. They are being held in a prison in Mirpur, south Pakistan.

Clive Stafford Smith, director of the legal charity Reprieve, has asked David Miliband, the foreign secretary, to contact the government of Pakistan "requesting that they fulfil their obligations under the convention against torture".

In a letter to Miliband, Stafford Smith describes how the two men were beaten for several days.

"Rehan was hung upside down from a hook ... then kicked and punched repeatedly. Over the next two weeks Rehan was subjected to hour upon hour of the most savage torture, including having cigarettes put out on his wrists," he says.

He said Hussain suffered equally brutal forms of torture, and after losing consciousness had to be taken to hospital.

Their ordeal began after Fazal Hussain, Naheem's father, who lives in Birmingham, visited his birthplace in Pakistan after he retired. He said he learned that his daughter's father-in-law had claimed his land. Before a meeting was held to discuss it two members of his extended family were shot.

The Hussains and Zaman were arrested at Dadyal police station in June 2004. Fazal was charged with conspiracy to murder; he was freed on bail in 2005, and returned to Britain.

Stafford Smith said statements by the Hussains and Zaman were "demonstrably false", and a ballistics report from Lahore said the bullets gathered were not shot from guns identified by Naheem and Zaman.

A Foreign Office spokesman said: "We are in discussions with Reprieve about how best to address the allegations."

Consular staff had visited both detained men, most recently on 24 March this year.